A Dance of Water and Air. Elemental Magicae, #1

Edmund is heir to the throne of Thalassa and a wielder of Water magic. Devoted to his kingdom and his duty to it, Edmund can do nothing but acquiesce to an arranged marriage with the queen of a neighboring kingdom. The marriage and the child it is required to produce will seal an alliance between Thalassa and Aither that is vital to Thalassa's safety, and far more important than Edmund's personal misgivings.
Arden is the younger brother of Aither's queen and a wielder of Air magic. Raised in the politics of the court to support his sister's rule, he understands the alliance is important to Aither, even as he worries about his sister marrying someone she's never met. When Edmund arrives in Aither to prepare for the wedding, Arden is tasked with helping him settle in at court. As they spend more time together, Edmund and Arden develop a close friendship, then stronger feelings, but with Edmund's wedding approaching, they must hide their feelings, even from themselves.
When someone tries to assassinate the queen, Edmund is blamed, and Arden rescues him before he can be executed for a crime he didn't commit. To prevent a war between their kingdoms and protect them from a dangerous enemy, Edmund and Arden will have to discover who wants to pit Aither and Thalassa against each other and mend relations between the two kingdoms as they evade those searching for them-all while finding a way to be together.